The function of pilot-training simulators is to reproduce as faithfully as possible the conditions of a true cockpit or flight deck. In particular, the simulated cabin is placed on a moving platform, so that the trainee feels the physical sensations corresponding to the movements of the aircraft. This platform is accelerated in rotation and translation and these movements are transmitted to it by hydraulic jacks provided with actuators whose construction is dependent on the degree of faithfulness with which the movements of the aircraft are to be reproduced.
In a known simulator arrangement, the simulator cabin is mounted on a platform supported by a device with six jacks placed on a base, the jacks essentially occupying the edges of an octahedron. The respective extension and retraction strokes of the jacks are controlled by a computer and determine in a relatively complicated manner the accelerations required for the platform. Each jack and the associated hydraulic and electrical actuating means form an assembly designated hereinafter by the term servo system.
The faithfulness of the accelerations obtained depends on a correct operation of the servo systems. It is often difficult to determine in a qualitative manner any deterioration in performance by simply observing the movements of the jacks and by interpreting the more or less subjective sensations felt by anyone in the cabin. In particular, the defective servo system can be relatively difficult to identify. Safety is also dependent on a correct operation of the jack. In order to guarantee the quality of the resulting movements, facilitate maintenance and contribute to safety, it would appear necessary to monitor the operation of each of the servo systems in an independent and accurate manner. Deterioration of the operating characteristics is mainly due to fouling of the servo valve settable for alternately advancing and retracting the jack together with its load, a rough-running point in the operation of the valve slide, or a defect in a transducer sensing the displacement of the jack.
This displacement is usually commanded by an externally generated position-control signal which is subtractively combined with a position-feedback signal sent back by the jack, the actuating means being responsive to an error signal which corresponds to the difference between the two position signals. Thus, the actuating means will generally comprise a subtractor working into an amplifier which transmits the amplified error signal to the servo valve. It is also known to provide monitoring means responsive to deviations between the position-feedback signal and a reference signal, derived from the position-control signal, for disabling the actuating means whenever these deviations exceed a predetermined tolerance range.
U.S. Pat. No. 3,724,330 describes a system with two electro-hydraulic servo valves whose transfer characteristics are duplicated by a pair of electrical models forming part of the aforementioned monitoring means. It has further been proposed (see U.S. Pat. No. 4,153,865) to compensate for the velocity-lag error of the jack by suitably modifying the monitoring signal. The generation of a velocity-proportional signal bucking the monitoring signal has also been described in German open application (Offenlegungsschrift) No. 2,431,912 published Jan. 15, 1976.